Britain’s spying agencies need more powers to read the contents of communications in the wake of the Paris terror attacks, David Cameron has said.

The prime minister’s comments suggest a Conservative government would bring in new intercept legislation in 2016 to make sure there is no form of communication that cannot be requested with a warrant signed by the home secretary.

This goes further than his separate plans to revive the communications data bill, known by its critics as the snooper’s charter, which was killed off by the Liberal Democrats last year.

Speaking in Nottingham, he said the intelligence agencies need more access to both communications data - records of phonecalls and online exchanges between individuals - and the contents of communications. This is compatible with a “modern, liberal democracy”, he said.

“There are two issues. One is communications data. That is not the content of calls. But who made which call, to which person, and when. This vital communications data is absolutely crucial not just to fight terrorism but finding missing people, murder investigations … What matters is that we can access this communications data whether people are using fixed phones, mobile phones or more modern ways of communicating on the internet … That is one piece of additional legislation that will be necessary.”

He then added that there was a “second thing is about accessing the content of a telephone call or another form of communication”. His comments signalled that the Conservatives will want to strengthen intercept legislation known as the Regulation of Investigatory Powers Act (Ripa), which is currently under review, using the justification of the Paris terror attacks.

Cameron said: “The same problem exists. Will we be able to access the content of communications as the internet and new ways of communicating develops? I have a very simple principle that will be the heart of the new legislation that will be necessary. In our country, do we want to allow a means of communication between people that even in extremis, with a signed warrant from the home secretary personally, that we cannot read? Up until now, governments have said no, we must not

“That is why, in extremis, it has been possible to read someone’s letter, to listen to someone’s call, to mobile communications … We have a better process for safeguarding this very intrusive power than probably any other country i can think of.

“But the question is are we going to allow a means of communications which it simply isn’t possible to read. My answer to that question is: no, we must not. The first duty of any government is to keep our country safe. The attacks in Paris demonstrated the scale of the threat that we face and the need to have robust powers through our intelligence and security agencies in order to keep our people safe.

“The powers that I believe we need, whether on communications data, or on the content of communications, I feel very comfortable these are absolutely right for a modern, liberal democracy.”

Tory sources confirmed that the plans would go further than the aborted snooper’s charter. One senior Conservative source said: “The coalition has only looked at communications data in the past. But obviously in coalition there are things that as a Conservative government we would like to do, that in coalition you have constraints on.”

Asked for more details about the proposed stronger powers to intercept online content, the source indicated there would be “more specifics” in the Conservative manifesto.

Cameron had already said on Sunday in an ITV interview that the Conservatives would bring back the communications data bill but his new comments suggest there would be separate legislation on content intercept next year.

Cameron made the case for the intelligence agencies to gain more powers to read private online communications despite existing public concerns about secret mass surveillance of UK citizens exposed by US whistleblower Edward Snowden.

This revealed the existence of a secret GCHQ programme called Tempora, which taps into transatlantic internet cables to monitor millions of items of correspondence.

Simon Hughes, the Lib Dem security minister, said the new powers suggested by the prime minister were unnecessary. He told the BBC: “Hold on to our liberties; don’t give them up in defence of the nation, specifically at a time where liberty is under attack. That’s the wrong message to send.

“[The security services] come to government with propositions on a regular basis, we’re responding again. The Liberal Democrats have no reason to believe that more powers are further needed.

“We’ll keep an open mind … The important thing is not to think that giving the authorities more power is necessarily the right thing to do. It may be about using the powers we have better, about having more people, making sure the borders are policed more correctly.”